Experience needed regarding FW Beater and Armor

[Runsler]

Those are some thoughts i well... didn't hope for (since they are basically saying i can't do this) but still wanted to see since i didn't had (all) of them.

Did some thinking and calculating on the weekend, just didn't have internet to post much about that, so there are slight updates:

currently i think about using titanium, 3cm thick, and everything 3cm wide. Also making it (almost) symmetrical with 24cm diameter, and thinking about adding two bolts per side into it as teeth. Still thinking about some counterweight for those bolts, since not having them right away would make the lathe work easier and less dangerous. but already having the counterweight without them could pose another problem there. This gives me about 5kg for the beater (pulley not included) and hopefully enough strength (will it?^^) at 0,02625 kg*m² MOI.


Okay, gonna try to post more structured.

1. yes, the idea was to make a pulley from two half-circles, with holes to fit a square shaft, bolted together over it. Thought this way i won't need a hub for it, and the bolts could get right through where the belt will go (after all, those are just kinda small holes, hopefully not making the belt wiggle) and possibly bolting it sideways into one of the big parts of the beater, if needed. Wanted to get that part watercut along with the beater. But honestly, i have no idea on how much force there is on that part, so i don't know if that would be sturdy enough.

2. Now i actually don't think getting the belt over the beater would be a big problem, as you can get it over the top part down to the shaft, and only then over the bottom part, making the longest needed the diagonal distance from inner shaft/beater to outer edge of the beater. That's 145mm. Adding some more, since the belt got some thickness, but still it's way less than 400mm So i think you might have just wanted to put it over from one side, and hopefully those described problems won't happen. (Still the part about using the right belts is really interesting. i see every kind of belt used on bots, but seldom someone says which one was better... they all seem to work just good enough or simply get off too often, nothing about not transmitting enough force.

3. Originally wanted to leave the middle part square, avoiding those problems with getting it round.
If i could get it round for a third bushing... i have to ask a friend of mine, he got the lathe and knows how to use it, really can't answer that by now. But polishing won't be a problem, beside the material (it will take a long time) it shouldn't be harder than some stuff i already did.

4. Yeah, getting the mount precisely made will be a big problem. I am used to having to work really precise, but... i am a dental technician, i usually work with stuff the size of teeth. If i could work that precise on this scale... at least i don't have the equipment to do it the way most people would do it. Have to put some more thinking into it if i can find a way, or if not... well, 30mm titanium square are still likely to bend? damned. Even have access to a CNC mill, but that one only can mill small stuff, too -.-
But good thing to know this kind of bushing is used and where it is used, somehow thought if i cut a bearing it won't make one exact circle again afterwards, possibly causing some problems.

5. Regarding the motor, i got one where the description says "1410W". thats maximum output and it didn't have a description for continuous output (or torque, as it is almost always with brushless ones), and 660kv, running on 5s.
So i calculated a bit with what i have, assuming a 3:1 reduction (somehow found that often reappearing in the build diaries)
The run amok calculator gives me a bit over 2300J of energy stored at full speed.
So if Watt is J/s, and even if there is a lot of energy lost on the way, i hoped that would be enough to get to reasonable speed within two seconds.
Also assumed only 1cm for the pulley width, if that is too thin i have to redo that a bit.
Also was going to put the pulley directly onto the bell of it, again not needing a hub (there are threaded holes to do this, after all).
The usual front would be the side mounted, calculated with letting the 2cm shaft just stick through my mounting plate, so i could have 2cm for mounting without extra size.
With the motor itself being about 5,5cm, i could have all that stuff in 7,5cm and leave a 2,5mm clearance on both sides. Not much, but within the 2-5mm you spoke of and making the numbers nice.


I know there is a lot of guessing in it from my side... but well, that's why i am asking here
If i see it right now, my biggest problems (aside from "is there enough weight for the rest of the bot?") will be the supporting of the middle, if that would be possible.
Well.. got half a day without much work for my brain to think about it now.

Anyway... really thanks a lot for all this thinking and those insights. Always better to get told "you can't do that and this is why" instead of just wasting money and time for something that couldn't work.

[overkill]

Titanium isn't a particularly good choice for a beater, its quite soft and the edges of the teeth will round over very fast, making it ineffective. It is possible to add hardened steel teeth to the tips but that is complicated. The main reason not to use titanium is simply cost; a slab that size will be thousands. Titanium will not run with bronze bushings due to galling https://en.wikipedia.org/wiki/Galling it will definitely need ball bearings for support and that rules out a central support bearing.

The belt problem: I drew up a quick sketch based on Alex's CAD and your description - its probably a bit different to what you have but good for discussion. With a 240mm diameter and 10mm high teeth, the shortest distance to fit a belt over is 162mm. With a 20mm thick beater the total path length is 364mm (I originally used a larger diameter and no off-set teeth to get the 400mm belt length).

What belt length is needed? If you went with a V belt, an A sized belt would be plenty strong enough. V belts all have a minimum diameter pulley they will work on and for A sized belts its around 38mm for the motor pulley. with a 3:1 ratio, the beater pulley is 114mm. placing the motor as far out as possible, the distance between shafts is around 95mm. Using an on-line belt length calculator, I get 443mm so you are in the clear. The 14mm pulley width is based on a 10mm wide belt and pulley cheeks that are 2mm wide at the top; you could go thinner but the pulley will be weaker.

Many builders use timing belts, which will work with smaller pulley diameters but I think they will slip too much with this design.

With the motor, maximum power is usually quoted for a 5 to 10 second period. The continuous power for 3 minutes could be anything but a safe bet is half the maximum power. A motor with a KV of 660 and running on 18.5 volts has a no load speed of 12,210 rpm. with a 3:1 reduction that gives you 4,070 rpm but with bearing friction and air resistance you will get closer to 3,000 rpm which is low by today's standards. With the massive weight of the beater it should still pack a fair punch. Based on the motor size that other builders need to get smaller & lighter beaters spinning without their motors catching fire, the motor you have will not be powerful enough and will likely overheat.

[overkill]

Just noticed another issue: Mounting a pulley to the bell of a motor is possible but not recommended for several reasons. The internal motor bearings are quite small and not intended to take side loads that the belt tension will put on them; I have had bearings literally explode from this problem. The belt tension will also strain the mounting screws and the further away the pulley is from the mounts, the more leverage it has. The motor base is relatively soft aluminium and the screws a small, so the threads will easily rip out of the base - again, I have several dead motors with this problem. Finally, the end bell can flex a tiny amount, causing the magnet to work loose. I had that happen to an expensive German motor, not a cheapo Chinese one.

I still can't visualise the shape of your design, but it has room for a 300 x 240mm cylindrical volume to fit the weapon. If you changed to a conventional beater and put the motor on the end, it will fit in the same space and all the tricky engineering problems will go away and costs will come down.

Back to your first post, one of the best fasteners for HDPE is the barrel bolt.

[Runsler]

Damn... didn't realize Titanium would be that complicated (regarding the bearings and stuff) and that expensive...
Also good remarks about the stability of the motor, didn't think about that. Also, i believed maximum power for two secounds would be good enough (to spin up once) and to hold it on a good level much less would be needed, anyway. But if you've seen those things blow of with similar characteristics, i'll take that experience and rethink that...
(Oh, and i know where we were different when talking about the length of the belt, you took total length, i was just looking for one way, since it is symmetrical)

Putting the Motor to one side would be an option, i just like symmetry, and believed with the pulley in the middle, it would be less likely to get cut off if anything attacks from the side. But seems like i would have to let go of that idea, not going to build something THAT expensive as a first try robot when it will most likely not even be really good and the expensive part is so "special" i can't possibly reuse it in a better/later build. quiet sad now, design was planned out so far with 1.6kg left for pulleys, wires and (some of the) bolts, without any pocketing and other weight-reducing stuff. Hoped that would be a reachable thing.

Also, that thing would have stored about 2000J with the last design. Not that much, but if the weight is there i somewhere read a slower weapon with the same energy stored would be stronger than a faster one, since it has more bite.



Anyway, even though i am a bit sad now, that was some really helpful advice, and i am glad you wrote all that stuff.
Regarding the "be nice policy", being nice to me is pointing out the problems with my plans, after all that's why i am asking here.

Well... need something different now. Weapon about the same diameter and (with axis) same length, but motor on one side not the middle, weight limit for the weapon 5kg, this time hopefully not from Titanium. Without the motor on the inside, maybe even a drum would be possible, only i have no idea how drums are made. just milled out from one solid block? Was originally going for a beater since the bot was going to be wide and a beater seemed easier to make... well... easy... yeah... also with a beater i would have had a name for it.^^
Okay, time to pause the weapon stuff for a moment until i did some more research and come up with a new idea and possibly show some scetches this time right away, so you could say what i have done wrong that time.


But for fastening HDPE, i have actually no idea what you mean by "barrel bolt". Google only give me those bolts to lock doors when i search for it, and my dictionary seems to fail me on this one. Do you have a picture of one, or maybe a different name?

[maddox10]

Barrel nut in Dutch is "dwarsmoer."

Try that in google images.

€70 a kilo is a decent price for Titanium. You can expect to find cheaper, but also a lot more expensive.

[R9000]

Yeah, you want a barrel nut, not a barrel bolt I think. It's just a small metal cylinder with a threaded hole in the side. You get them quite a lot in IKEA furniture and the like.

[Runsler]

Guess i've found "a lot more expensive" on my quick search.
Anybody know what K-Cut or any watercutting service like that takes?
And if they do calculate for the finished product, or the sheet it is cutted from?

for 5kg*70€/kg it would be 350€... expensive, but payable. add some bolts as teeth to work around titanium being too soft it could actually work... maybe i don't have to start over again that much.
But i will try some designs with the motor to one side, too... even though i don't like to have a "weak" side, but seeing all these troubles with the mid solution i should at least test it.
Regarding the galling, i've seen some plastic bushings. they seem to be teflon coated, are even lighter and come in the right size.
Would those be working, or would i still have the same problem, and possibly even one more since they are not made from metal?

Also got me thinking... would it cause yet again new/different problems to make the axis non-rotating, and put the bushings actually into the weapon part? would have to strengthen that part, since obviously the material gets a lot thinner when i put a hole into it, and the pulley would have to be directly connected to the weapon instead of to the axis. could possibly even just make a ring as a pulley instead of an actual disc. This way i could make the axis from steel (that won't be soooo much weight), the weapon possibly from titanium, but won't have the galling, since the bushing stands relative to the titanium still, and only rotating to the steel (which should be no problem, i think).



Ah, and thanks, google actually found the dwarsmoer. now i recognize them, used those a few times when working with wood. Logical they would work with HDPE, too.

Another idea would be thin plates with a threaded hole inside working quiet the same way, only they are thinner and might be lighter. But the thread would also be more likely to get ripped out.
Got a small plate HDPE in the post today, will do some testing to that one looking what is working the best.

Oh well...
So much to rethink and research and so little time and knowledge beforehand

[maddox10]

Forget plastic bushings. Those are for light loads only.

It also seem that I'm one of the few that uses sinterbronze bushings on the spinners (Valkiri for example, that's 7.8Kj of spinner)

Titanium for a spinner. Not only is the base material a lot more expensive, but the machining is a lot tougher(or expensive) and the vurnability is another matter. Securing the teeth/hammers/cutters isn't easy.

It's more cost effective to make a smaller monoblock beater in decent steel like high grade hardox.

Nick has a lot of experience in compounded spinners, bars and beaters, and in his last machines, it's all monoblock.
I would defer to the master if it comes to spiny things.

[Runsler]

Okay, so no plastic bushings.
But what else (if not bronze) do others use? ball bearings also don't seem good.
Also, how about that idea with the bushings in the rotating part (let's say bronze bushings, no matter what is rotating there) on a fixed steel axis? which problems would that cause?

Since i need to redesign now, anyway, i could as well try to go a complete different way for the weapon design.


The problem is with a smaller diameter, my complete design and idea is useless. The important thing about that is, that it is wedge-shaped and invertible, with a vertical spinner to the back of the wedge. now if i make the diameter of the spinner smaller, it is hanging somewhere in the air (at least from one side), and a lot of ground-clearance isn't something too good in a spinner, i think.

So i have to stay roughly with that. And with titanium seemingly a bad idea, and Ali possibly even worse (at least when it comes to durability, sure not cost- or machining-wise) i have to search for a way to make a steel spinner with that diameter.

Or, if that is impossible (as it seems) even have to downgrade from such a wide weapon to a thinner one, like a disc. But with that, i'd have to redesign the mounts (or have a disc on a 30cm axis o.0), so yeah, a lot of work to do for me.

[Andy]

Could you not create a steel beater of a smaller diameter and adjust/redesign the bulkheads accordingly? If the bulkheads are still in the design stage im sure itll be worth the extra bit of time